Any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses. This means that the wavelength of the light emitted from the object is getting longer as the object moves away from the observer. The notion of an event horizon was originally restricted to black holes; light originating inside an event horizon could cross it temporarily but would return. Later, in 1958, a strict definition was introduced by David Finkelstein as a boundary beyond which events cannot affect any outside observer at all, encompassing other scenarios than black holes. This strict definition of EH has caused information and firewall paradoxes; therefore Stephen Hawking has supposed an apparent horizon to be used, saying "gravitational collapse produces apparent horizons but no event horizons" and "The absence of event horizons mean that there are no black holes - in the sense of regimes from which light can't escape to infinity. "